The definition comes from when you go to your car and the driver unlocks the doors but you pull the handle to soon. What happens from this is your door remains locked but every other door in the car is unlocked. People refer to this as screwing the pooch.

Narrator: Person one and person two are outside of their car as person one unlocks the car doors. Person two accidentily opens the car door too soon making his door stay locked.
Person one: Dude, why you be screwing the pooch!
person two: ya well...ur mom!!!